1. Field of the Invention
The present invention relates to photoelectric sensors which project light toward an object to be detected and receives reflected light or transmitted light thereof, thereby detecting presence or absence and information such as color of an object to be detected; and more particularly, relates to a main body unit of an optical fiber photoelectric sensor and the optical fiber photoelectric sensor, which projects light toward the object to be detected via an optical fiber.
2. Description of the Related Art
Conventionally, there has been used an optical fiber photoelectric sensor which includes a fiber sensor head to which an optical fiber cable is connected and a main body unit which projects light to a detection area via the fiber sensor head and receives transmitted light or reflected light thereof in order to detect presence or absence of an object, dimensions of the object, a distance between the objects, a position of the object, a shape of the object, colors, and the like. A light projecting unit which is for emitting light to an optical fiber and a light receiving unit which receives light from the optical fiber are provided inside the main body unit.
A light emitting diode (LED) is generally used as a light projecting unit. The LED includes an LED chip which is mounted on the surface of a substrate connected to a lead wire and a translucent resin sealing layer which is formed so as to cover the LED chip. A light projecting side optical fiber is butted to the resin sealing layer of the LED, and light emitted from the LED chip is guided to the inside of the light projecting side optical fiber.
Optical coupling efficiency with respect to the light projecting side optical fiber of a light emitting element is used as a parameter showing detecting capability of the optical fiber photoelectric sensor. The optical coupling efficiency mentioned herein is the parameter showing proportion of light actually radiated to an area to be detected for the total amount of emitted light emitted by the light emitting element. When the optical coupling efficiency is small, an amount of projected light contributing to detection is small and therefore high detecting capability cannot be obtained. The amount of emitted light needs to be increased by flowing a large current to the light emitting element in order to obtain high detecting capability. However, in this case, it becomes a problem in that specifications of the light emitting element are limited, consumption energy is increased, and life span of the light emitting element becomes short.
In order to enhance the above-mentioned optical coupling efficiency, as shown in FIG. 9, it is conceivable to arrange an LED chip 41 serving as the light emitting element near an incident end surface of a light projecting side optical fiber 3. This arrangement makes it possible to make most of light emitted from an LED chip 41 incident on the optical fiber. However, light P2 emitted at an incident angle at a certain degree or more with respect to an optical axis of the LED chip 41 is radiated to the outside of the optical fiber after the light is incident on the optical fiber. This is because that an incident angle α made by the light P2 to the optical fiber does not satisfy reflecting conditions in the inside of the optical fiber; and therefore, the light P2 is emitted to the outside without reflecting on the inside. Consequently, even if the light emitting element is merely arranged near an end surface of the light projecting side optical fiber 3, an amount of light incident on optical fiber from the light emitting element increases; however, light loss at the optical fiber becomes large by just that much; and therefore, resultant optical coupling efficiency cannot be enhanced.
Furthermore, as shown in FIG. 10, it is conceivable to provide a condenser lens 42 in front of an LED chip 41 (for example, see Japanese Unexamined Patent Publication Nos. S59-180515 and H9-307144). According to such a configuration, it is possible to make light radially radiated from an LED chip 41 effectively incident on an end surface of a light projecting side optical fiber 3 by a function of the condenser lens 42. However, generally, the LED chip 41 radially emits light from an emission surface; and therefore, of the light radiated from the LED chip 41, light P2 emitted at an angle at which light is not incident on the condenser lens 42 is not subject to the converging function of the condenser lens 42; as a result, the light P2 is not incident on the optical fiber. Therefore, when the LED chip 41 is arranged close to the condenser lens 42 in order to make the light P2 incident on the optical fiber, an incident angle α made by the light P2 to the optical fiber becomes large; and light loss at the optical fiber becomes large after all.
In order to solve the above-mentioned problems, it is conceivable to provide a reflecting surface 40 which reflects light emitted to a direction not incident on a condenser lens 42 as shown in FIG. 11, thereby making all light emitted from an LED chip 41 incident on an optical fiber. However, even with such a configuration, light P2 reflected by the reflecting surface 40 is incident on the optical fiber at a relatively large incident angle α; and therefore, in the case where the incident angle α does not satisfy reflecting conditions of the optical fiber; after all, light P2 is radiated to the outside of the optical fiber. Consequently, even by adopting such a configuration, the light emitted from the LED chip 41 cannot be sufficiently coupled to the optical fiber.
Furthermore, different from the above-mentioned problem, generally in an optical fiber photoelectric sensor, there is a problem in that when light is not uniformly incident on an incident end surface of a light projecting side optical fiber 3, a deviation is generated in an optical axis of light emitted from the other end surface on the projecting light side is deviated. FIG. 12 is a view showing a light beam flow in the case where a central axis of an LED chip is slightly deviated from an optical axis of a condenser lens 42. In this case, distribution of the light incident on the optical fiber is converged to a right side region with respect to the optical axis of the condenser lens 42; and therefore, a distribution of an amount of incident light becomes non-uniform. In order to make the light uniformly incident on the light projecting side optical fiber 3, an LED chip 41 and an optical axis of the condenser lens 42 need to be correctly positioned; and attachment needs to be made so that a central axis of the light projecting side optical fiber 3 is correctly coincided with the optical axis. However, generally, correct positioning of the axes is not easy; and therefore, this causes difficulty in manufacturing and an increase in manufacturing cost.